1. Field of the Invention
This invention relates to the preparation of polymer foams and is more particularly concerned with novel catalyst combinations for the trimerization of polyisocyanates to polyisocyanurates and their utilization in the preparation of polyisocyanurate foams.
2. Description of the Prior Art
Rigid polyisocyanurate foams having high resistance to flame and heat as well as excellent thermal insulating capacity are known in the art. The prior art discloses methods for preparing such foams by reacting an organic polyisocyanate with a trimerizing catalyst in the presence of a blowing agent, and a minor proportion (usually less than 0.5 equivalent per equivalent of polyisocyanate) of a polyol; see for example U.S. Pat. Nos. 3,516,950, 3,580,868, 3,620,986, 3,625,872, and 3,725,319. The process described in U.S. Pat. No. 3,745,133 discloses the use of a combination of an epoxide and a tertiary amine as cocatalysts.
Certain quaternary hydroxyalkyl tertiary amine bases have been recognized as possessing catalytic activity for polyurethane or polyisocyanurate formation; see U.S. Pat. Nos. 3,010,963, 3,892,687, B 497,194 (published Feb. 3, 1976), and B 490,946 (published Feb. 17, 1976).
The prior art has also recognized the inherent problems in the combination of a polyurethane forming reaction with a polyisocyanurate forming one, particularly with regard to the difference in the two reaction rates and methods of overall rate control. U.S. Pat. Nos. 3,896,052 and 3,903,018, whose disclosures are hereby incorporated by reference, provide catalyst combinations which overcome these difficulties.
A particularly difficult catalysis problem is encountered in the preparation of polyisocyanurate foam laminate board stock. It requires a foam rise profile characterized by a long extended cream time followed by a rapid rise and cure out. U.S. Pat. No. 3,896,052 provides catalyst combinations which meet these requirements. However, the optimum catalyst mixtures disclosed therein call for the combination of amide and glycine salts in conjunction with a tertiary amine trimerization catalyst and a monomeric epoxide component. The reactivity between the other foam ingredients, particularly amines and epoxides, precludes their being preblended prior to the actual polymerization stage. Accordingly, the number of reactant streams going to the reaction zone must be at least three in order to accommodate separate polyisocyanate, polyol plus amine, and epoxide ingredients. The majority of foam laminate machinery presently available is equipped for only two component lines to the mixing zone.
The present invention provides a catalyst combination which can be used in a two component system, i.e. a system in which the reaction components can be preblended to form just two reactant streams. Surprisingly, it has been discovered that the epoxide and tertiary amine components of the prior art catalyst combinations can be replaced by a single component, namely one or more hydroxyalkyltrialkylammonium carboxylate salts. Moreover, the substitution of this minor amount of ammonium carboxylate for the substantial amount of epoxide and amine effects an economic advantage, and gives rise to excellent foam exotherms and rise characteristics which result in extended foam cream times which are immediately followed by rapid rise and cure out periods.